Sensitivity and vulnerability methods can be applied to a variety of hydrogeologic settings, whether or not they contain specifically identified aquifers. However, some methods are best suited to assess groundwater within aquifers, while others assess groundwater above aquifers or groundwater in areas where aquifers have not been identified.

Intergranular media systems, including alluvium and terrace deposits, valley fill aquifers, glacial outwash, sandstones, and unconsolidated coastal plain sediments are characterized by intergranular flow, and thus generally exhibit slower and more predictable groundwater velocities and directions than in fractured media. Such settings are amenable to assessment by the methods described in this guide. Hydrologic settings dominated by fracture flow or flow in solution openings are generally not amenable to such assessments, and application of these techniques to such settings may provide misleading or totally erroneous results.

The methods discussed in this guide provide users with information for making land- and water-use management decisions based on the relative sensitivity or vulnerability of underlying groundwater or aquifers to contamination. Most sensitivity and vulnerability assessment methods are designed to evaluate broad regional areas for purposes of assisting federal, state, and local officials to identify and prioritize areas where more detailed assessments are warranted, to design and locate monitoring systems, and to help develop optimum groundwater management, use and protection policies. However, some of these methods are independent of the size of the area evaluated and, therefore, can be used to evaluate the aquifer sensitivity and vulnerability of any specific area.

Many methods for assessing groundwater sensitivity and vulnerability require information on soils, and for some types of potential groundwater contaminants, soil is the most important factor affecting contaminant movement and attenuation from the land surface to groundwater. The relatively large surface area of the clay-size particles in most soils and the soils' content of organic matter provide sites for the retardation and degradation of contaminants. Unfortunately, there are significant differences in the definition of soil between the sciences of hydrogeology, engineering, and agronomy. For the purposes of this guide, soils are considered to be those unconsolidated organic materials and solid mineral particles that have been derived from weathering and are characterized by significant biological activity. In the United States, these typically include unconsolidated materials that occur to a depth of 2 to 3 m or more.

In many areas, significant thicknesses of unconsolidated materials may occur below the soil. Retardation, degradation, and other chemical attenuation processes are typically less than in the upper soil horizons. These underlying materials may be the result of depositional processes or may have formed in place by long-term weathering processes with only limited biological activity. Therefore, when compiling the data required for assessing groundwater sensitivity and vulnerability, it is important to distinguish between the soil zone and the underlying sediments and to recognize that the two zones have significantly different hydraulic and attenuation properties.

1.1 This guide covers information needed to select one or more methods for assessing the sensitivity of groundwater or aquifers and the vulnerability of groundwater or aquifers to water-quality degradation by specific contaminants.

1.2 This guide may not be all-inclusive; it offers a series of options and does not specify a course of action. It should not be used as the sole criterion or basis of comparison, and does not replace professional judgment.

1.3 This guide is to be used for evaluating sensitivity and vulnerability methods for purposes of land-use management, water-use management, groundwater protection, government regulation, and education. This guide incorporates descriptions of general classes of methods and selected examples within these classes but does not advocate any particular method.

1.4 Limitations—The utility and reliability of the methods described in this guide depend on the availability, nature, and quality of the data used for the assessment; the skill, knowledge, and judgment of the individuals selecting the method; the size of the site or region under investigation; and the intended scale of resulting map products. Because these methods are being continually developed and modified, the results should be used with caution. These techniques, whether or not they provide a specific numeric value, provide a relative ranking and assessment of sensitivity or vulnerability. However, a relatively low sensitivity or vulnerability for an area does not preclude the possibility of contamination, nor does a high sensitivity or vulnerability necessarily mean that groundwater or an aquifer is contaminated.

1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

1.7 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied without consideration of a project's many unique aspects. The word “Standard” in the title of this document means only that the document has been approved through the ASTM consensus process.